The present invention relates to a measuring probe, also known as an analogue or proportional probe, and which may, for example, be used on a coordinate positioning machine, such as a coordinate measuring machine or machine tool, in order to measure the shape, form, or dimensions of an object.
A measuring probe typically includes a relatively fixed structure, which usually has the form of a housing, and a relatively movable structure the function of which is to retain a workpiece-contacting stylus, and is therefore frequently known as a stylus holder. The stylus holder is suspended within the housing by a mechanism which provides relative motion of one relative to the other. Relative displacement of the stylus holder and housing, (and therefore changes in relative displacement during relative motion) is detectable by one or more transducers, which are usually mounted to, or otherwise provided within the housing of the probe.
In use, the probe and a workpiece under inspection are mounted to relatively movable parts of the machine (the probe being mounted to the machine by the housing, thus enabling the stylus to move freely), and the machine is operated to bring the stylus into contact with a surface of the object in respect of which form, dimension, or contour information is required. Whilst the stylus is in contact with the surface, the transducer outputs from the probe are directly indicative of the relationship between the part of the machine upon which the probe is mounted and the surface under inspection. The position of a point upon the surface relative to a fixed reference point on the machine may thus be determined from signals indicative of the relative position of the two relatively movable parts of the machine, and the transducer outputs of the probe.
One known form of analogue probe is disclosed in European Patent No. 544854, and has a housing relative to which a stylus holder is suspended by a mechanism which includes a first flexible diaphragm connecting stylus holder to an intermediate member, which is then in turn connected to the housing via two further mutually parallel flexible diaphragms. A similar design of probe is disclosed in European Patent 426492. Both prior art configurations of probe provide releasable mounting of a stylus to the stylus holder by means of magnets and mutually engageable elements forming a repeatable kinematic location. This enables a user to alter the configuration of stylus in dependence upon a particular inspection task to be undertaken with the probe.
One aspect of the present invention provides a modular configuration of probe, in which the suspension mechanism connecting the stylus holder to the housing is provided within a suspension module that is releasably and repeatably connectable to a transducer module containing one or more sensors of the probe transducers. In accordance with this aspect of the present invention, one configuration of stylus may be exchanged for another by exchanging the suspension module. Also by arranging that the mutually engageable elements locating the suspension module on the transducer module are situated on the housing of the suspension module and transducer module respectively, rather than on the stylus holder as is the case in the prior art, the inertial mass carried by the suspension mechanism is reduced. In this way the sensitivity of the probe is increased.
Any suitable suspension mechanism may be provided within the suspension module, such as, for example, a series of parallel leaf springs, as disclosed in U.S. Pat. No. 4,084,323, a series of linear bearings as disclosed in U.S. Pat. No. 5,088,208, an assembly of linkages, each of which is connected both to the stylus holder and the housing as shown in U.S. Pat. No. 4,523,383, or one or more flexible diaphragms, such as disclosed, for example in U.S. Pat. No. 5,345,689 or U.S. Pat. No. 4,158,919.
According to another independent aspect of the present invention a measuring probe comprises a housing having an axis, a stylus holder extending along the axis and to which a stylus is connectable, and a suspension system for the stylus holder, wherein the suspension system comprises at least a pair of substantially planar diaphragms connected between the stylus holder and the housing and lying in first and second parallel planes orthogonal to, and spaced apart along, the axis, both of said diaphragms allowing limited axial movement of the stylus holder, and at least one of the diaphragms being sufficiently flexible in its plane to allow limited transverse movement of the stylus holder in the plane of said at least one diaphragm.
In order to achieve the required flexibility of the, or each of said flexible diaphragms, one or more channels are provided through the respective diaphragms.
In one embodiment, the diaphragms are identical and the channels are of a substantially spiral configuration, each diaphragm having three such channels circumferentially offset by 120xc2x0. By virtue of such an arrangement, a relatively simple and friction free suspension mechanism is afforded together with a relatively compact construction.
With such a suspension mechanism, the stylus holder is displaced relative to the housing, upon the application of a force to the stylus, either linearly in the direction of the probe axis, or in a tilting manner, in planes substantially transverse to the probe axis. Detection of the linear axial displacement and the tilting displacement of the stylus holder will provide an indication of the position of the stylus sensing tip, for a given length of stylus. Because the suspension mechanism provides tilting action, different lengths of styli will produce, for a given linear deflection transverse to the axis at the stylus tip, a different degree of tilting of the stylus holder relative to the probe housing. Typically this is taken into account by calibration of the machine upon which the probe is to be used.
Alternatively the transducer system used may be configured to compensate for different lengths of stylus producing different tilting angles for a given displacement transverse to the axis at the stylus tip.
The transducer system used is preferably an optical system which includes a light source which emits a beam of light incident upon an optical feature mounted to the stylus holder, which light beam is then passed on to a photosensitive detector that generates an output in dependence upon the incident position of the light beam on its photosensitive surface.
The optical feature is preferably provided by a reflective or refractive element that interacts with an incident beam to reflect or refract the beam upon interaction with the feature by an angle determined by one or more optical parameters of the feature. Thus, by varying the optical parameters which determine the reflective or refractive interaction, different tilting angles of the stylus holder (corresponding to the different tilting produced by a given tip deflection for different stylus lengths) may result in the same incident position of the reflective or refracted beam on the sensitive detector, thereby generating a constant output for a constant tip deflection transverse to the axis which is independent of the stylus length.
In one embodiment, the optical feature is provided by a mirror whose curvature is dependent upon the stylus length: longer stylus lengths having a greater curvature of the mirror in order to ensure that a smaller tilting displacement produces the same deflected angle by virtue of reflection of the incident light beam at the curved mirror.
In an alternative embodiment the optical feature is provided by a Fresnel lens, for example.